Process of making a metal article of precise dimensions and the article produced thereby



Patented 17, 1 936 UNITED. STATES PATENT- OFFICE:

PROCESS ORMAKING A METAL ANTICLE F PRECISE DIMENSIONS AND THE PRODUCED THEREBY ARTICLE Henry A. De Cleveland, Ohio, assignor, by

mesne assignments, to The Nitralloy Corporation, a corporation of Delaware v No Drawing. Continuation of application Serial No. 273,432,, April 27, 1928. This application February 20, 1932, Serial No. 594,389

12 Claims. (01. us-1s) 10 pressure or other severe stresses, whereas others,

such as forming dies, must sustain considerable loads. All such instruments, however, require a hard, wear-resisting surface, and it is highly desirable that such surface be of a stable, non-corl5 rosive character.

In the manufacture of such devices it has long been the practice to make them of hardened or case carburized steels. With the ordinary processes the maximum degree of hardness which can be obtained does not exceed 750 Brinell, and it is very diflicultto prevent deformation and distortion of the pieces due to the quenching operation necessary in the hardening process.

It is known that certain steels can be surface hardened by the so-called nitriding processas described in United States Patent No. 1,487,554, issued to Adolf Fry on March 18, 1924. By this process, I find that machined and heat treated pieces can be hardened at such low temperatures 30 as to retain the physical properties imparted to the cores thereof prior to nitriding, and if proper precautions are taken, may. be done without any distortion or deformation. Furthermore, the hardness which can be imparted to the surface a ranges between 900 and 1100 Brinell.

The steels most suitable for this process are the so-called Nitralloy" or aluminum bearing steels as described in another United States Patent No. 1,649,398 issued to Fry on November 15, 1927. In 0 using these steels, however, I have found that the best results are obtained by dividing them into two series, to wit: First, an aluminum bearing steel of medium carbon content for those instruments which do not require a core hardness of more than a 300 Brinell; and second, an aluminum bearing steel of higher carbon content for those instruments which require a core hardness in excess of 300 Brinell.

In the first series, a carbon content of from 0.15% to 0.50% is recommended, while for the 50 second series, steels containing from 0. 50% to 2.0% carbon (the other alloying elements being in proportion) are recommended.

For example, where the instrument does not 6 require a core hardness in excess of 300 Brinell,

. Balance iron a steel of about the following composition will prove satisfactory:

Per cent, 0.25-0.35 1.50-1.75 0.50-0.60 0.20-0.30 0.15-0.25 1.00-1.25

Car n Chromium Manganese -1 sili con V Molybden Aluminum Balance iron Such a steel annealed will give about 170 Brinell, andwhen quenched in oil from 1650 degrees F. and tempered at about 1050 degrees F. will attain a Brinell hardness of about 300.

When an instrument having a. greater core hardness is required, a steel of the following composition will be found satisfactory:

Per cent Carb n Chromium Manganese Silicon Molybdenum Aluminum Such a steel annealed will give about 200 Brinell andwhen quenched in oil from 1600 degrees F. and tempered at 1050. degrees F. will attain a Brinell hardness of about 480. This same steel, if heated to 1600-1700 degrees F. and air cooled, will attain a Brinell hardness of 600. In any case the hardness in either series will vary somewhat with different tempering heats.

In the manufacture of instruments by my process, it should always be borne in mind that nitriding subjects the steel to prolonged heating at about 900 degrees F., and for this reason any tempering or core hardening which is done prior to nitriding should be done at a temperature in excess of 900 degrees F. Fm'thermore, it is obvious that no case, however hard in itself, is ever stronger than the core which supports it, and that the case itself on account of its extreme hardness must be somewhat less elastic than the steel before hardening. Therefore, it is quite essential that a supporting core of sufiicient hardness should be provided to prevent the casefrom being stressed to the cracking point. Hence it is important that steel with the proper carbon content to provide the requisite hardness be selected.

A plug gauge, for'example, sustains no load, and, therefore, low carbon steel annealed and nitrided can be used. On the other hand, praccore hardness of from 400 to 500 Brlnell in order to withstand the pressure developed when the wire is drawn through he pening therein.

After having selected a steel in which the required physical properties of the core may be developed by suitable heat treatment, a working surfacelmay be applied thereto which will be ex tremely hard, extremely resistant to wear, and absolutely rustle'ss in so far as atmospheric, fresh and salt water corrosion is concerned, thereby combining all these desirable requisites in the same unit.

In the manufacture of instruments of precision which require only-annealing, I proceed as follows: The part is first rough formed by machining or otherwise from steel containing elements adapting it to be nitrided and which preliminary to the rough forming or machining operation preferably has been annealed. Where the instrument is of simple section it will not be necessary to reheat the 8 1 formed part, but where a complicated section is involved it should be thoroughly reheated to a temperature suflicient to relieve any internal strains, developed in forging or machining. Afterroughforming of asimple section or after the rough forming and reheating of a more complicated section, it is finished by machining and grinding to the precise dimensions required, and is then nitrided to harden the surface. I

During the nitriding, while there is no distortion or deformation, there is a very slight growth due to the absorption of nitrogen. This growth is only about 0.0003 to 0.0010 inch depending upon the duration of the treatment; Where this growth exceeds the tolerance allowed, the instrument may, thereafter, be reduced to exact size by a slight lapp ng operation. Otherwise, allowance may be made for this slight growth in the fine machining and grinding, and the lapping omitted. I

Where the instrument requires heat treatment for hardening the core, I proceed as follows: The device is first rough formed by machining or otherwise from steel containing elements adapting it to be nitrided andwhich preferably has been previously brought to the annealed state. The rough formed part is then heat treated for the p pose of developing the required physical properties by subjecting to a hardening heat treatment followed by tempering. If the section is of simple character it need not be reheated after the rough forming operation and before subsequent working or heat treatment operations, but, "otherwise, it should be reheated to relieve any internal'strains set up during the rough forming. After reheating to relieve internal strains followed by heat treatment to develop the desired core hardness, or, if a simple section, after rough and heat treat ment to develop the require' cl physical properties,

the instrument is finishand to the exact required dimensions. It is then nitrided, and may thereafter belapped if the growth during the nitriding exceeds the tolerance allowed and no allowance for the growth has been made in the finish grinding. The reheating treatment to relieve internal strains set up during machining or other working operations may constitute a full anneal or a normalizing treatment when it precedes the heat treatment for developing core hardness. If it occurs after the hardening heat treatment, then the' temperature in such reheating treatment should not be carried above the 2,038,027 1 ticehasshownthatawiredrawingdierequiresa drawing temperature employed in the hardening heat treatment. q

Sometimes a thin film of oxide appears on the surface of the nitrided article. If objectionable, this film can be readily removed with fine emery cloth.

From the foregoing it will be apparent that instruments of precision may be made in the manner described to exact desired form and di-- mensions and without fear 0 distortion during hardening.

The case produced by a ninety hour nitriding treatment will be about 0.031 inch deep. The outer half is extremely hard, and the case merges gradually into the core without any line of demarcation. For this reason a nitrided instrumentcannot suffer from. spelling or exfoliation as so frequently occurs in parts poorly carburired. Growth or distortion greatly in excess of that normally to be expected, as stated above, is an indication that the preliminary forging, machining and reheating operations have not been performed with suilicient care. All strains set up by forging or heavy machining must be relieved by an appropriate drawing or annealing heat treatment, and all scale, resulting from forging or heat treatment, should be removed from surfaces that are to be subsequently nitride hardened.

This is a continuation of my application Serial No. 273,432 filed April 27, 1928.

I claim:

1. The process of making a metal article of precise dimensions having an extremely hard, rust and wear-resisting surface which comprises rough forming the article to the approximate dimensions from. steel containing elements adapting it to be nitrided, heat treating and tempering the article to provide the necessary core hardness.

grinding to precise dimensions; and thereafter nitriding to harden the sin-face portions.

2. The process of making a metal article of precise dimensions having an extremely hard, rust and wear-resisting surface which comprises rough forming the article to the approximate dimensions from steel containing elements adapting it to be nitrided, heat treating and tempering the article to provide the necessary core hardness. reheating to a temperature sufliciently high to relieve internal strains but below the drawing temperature, grinding to precise dimensions, and thereafter nitriding to harden the surface.

3. The process of making a metal article of precise dimensions having an extremely hard, rust and wear-resisting surface which comprises rough forming the. article to the approximate dimensions from steel containing elements adapting it to be nitrided, heat treating and tempering the article to provide the necessary core hardness, grinding to precise finished dimensions, nitriding to harden the surface portions, and thereafter lapping to exact dimensions.

4. The process of making a metal article of precise dimensions having an extremely hard, rust and wear-resisting sin'face which comprises rough forming the article to approximate dimensions from steel containing elements adapting it to be nitrided, heat treating and tempering the article to provide the necessary core hardness, reheating to a temperature sufllciently high to relieve internal strains but below the drawing temperature, grinding to exact finished dimensions, nitriding, and thereafter lapping to exact finished dimensions.

5. Those steps in the manufacture of a metal article oi precise form and dimensions havinga nitrided case of extreme hardness and resistance to rust and wear, which comprises heat treating and tempering the article at a temperature above about 900 degrees F. to harden the core, reheating to a temperature sumciently high to relieve internal strains but below the drawing tempera- L ture, and thereafter nitriding.

6. Those steps in the manufacture of a metal article of precise form and dimensions having a nitrlded case of extreme hardness and resistance to rust and wear, which comprise heat treating and tempering the article at a temperature above about 900 degrees F. to harden the core, and thereafter nitriding to harden the surface.

'L'The method of nitriding articles made from steel alloys, which comprisesheating the articles to a temperature above their upper critical point, quenching the articles, drawing the articles at a temperature below their lower critical point, and

then nitriding the articles, the object of said heat treatment being to increase the depth and hardness of the nitrided case to a degree greater than is obtainable when the articles are nitrided without being thus previously heat treated.

8. The method of nitriding articles made from steel alloys, which comprises heating the articles to atemperature of about 1650 F., quenching the articles, drawing the articles at a temperature of about 1050 F., and then nitri'dingthe articles, the object of said treatment being to increase the'depth and hardness of the nitrided case to a degree greater than is obtainable when thearticles are nitrided without being thus previousiy heat treated.

it. As a' new article of manufacture. an instrument of precision having a nitride hardened suriace and composed 01 steel containing carbon from about 0.15% to about 2.0% with aluminum from 0.5% to 2.0%; said instrument being characterized by having its core zone hardened by heat treatment.

10. As a new article of manufacture an instrument oi. precision required to withstand high pressures and composed of steel having its core zone hardened by heat treatment, and in its marginal layers a rust and wear-resisting nitrided case.

11. An article which in its normal use is subjected to high pressures or shocks in at least a part thereof; said article being composed, at least in those parts which are subjected to such high pressures or shocks, a steel alloy containing chromium and aluminum and which is susceptible of acquiring such a high hardness in its core portions when subjected to a hardening heat treatment that the Brinell hardness in the core does not decrease below about 300 when said hardened steel alloy is reheated to around 1050v F., said article having its core hardened by heat treatment as aforesaid and having at least the parts thereof which are subjected to such high pressures or shocks surface hardened by nitriding.

12. An article which in its normal use is subjected to high pressures or shocks; said article consisting of a steel alloy containing chromium and aluminum and having a nitride hardened surface and a heat treated core portion, said core portion having a hardness of about 400 or more Brinell.

HENRY A. DE FRIES. 

